MC9S12XD256MAL Datasheet, PDF (119/1348 Pages) Freescale Semiconductor, Inc – Freescale’s Scalable Controller Area Network

Chapter 3
Pierce Oscillator (S12XOSCLCPV1)
3.1 Introduction
The Pierce oscillator (XOSC) module provides a robust, low-noise and low-power clock source. The
module will be operated from the V_DDPLL supply rail (2.5 V nominal) and require the minimum number
of external components. It is designed for optimal start-up margin with typical crystal oscillators.
3.1.1 Features
The XOSC will contain circuitry to dynamically control current gain in the output amplitude. This ensures
a signal with low harmonic distortion, low power and good noise immunity.
• High noise immunity due to input hysteresis
• Low RF emissions with peak-to-peak swing limited dynamically
• Transconductance (gm) sized for optimum start-up margin for typical oscillators
• Dynamic gain control eliminates the need for external current limiting resistor
• Integrated resistor eliminates the need for external bias resistor
• Low power consumption:
— Operates from 2.5 V (nominal) supply
— Amplitude control limits power
• Clock monitor
3.1.2 Modes of Operation
Two modes of operation exist:
1. Loop controlled Pierce oscillator
2. External square wave mode featuring also full swing Pierce without internal feedback resistor
MC9S12XDP512 Data Sheet, Rev. 2.21
Freescale Semiconductor
119